In recent years, high quality colour printers have become a norm. Two significant and related factors led to this norm, namely improvements in accuracy in colour reproduction and improvements in resolution. For ink jet printers, typical resolutions are 1200 dpi or higher, which translates into a printer ink dot size (and separation) of 20 microns or less. In many systems, the ink jet printer may overprint regions multiple times to help minimise the effect of printer defects, such as blocked printer head nozzles. The optical density of a printed colour can be sensitive to the precise value of the displacement between overprinted regions. This means that (for high quality at least) the exact displacement of the printer head between overprints must be controlled or calibrated.
A number of approaches have been proposed for calibrating the movements of the printer head relative to the medium being printed on in a precise manner. Several principal approaches can be summarised as follows:                Measure (using the human eye, or more recently an optical sensor) optical density of an overlapping, interlaced dot pattern (also known as complementary dot patterns);        Measure alignment of a series of lines (visually inspection using the Vernier effect);        Measure alignment of an interlaced series of lines (Vernier effect using optical sensor); and        Measure (using a scanner) individual positions of sparse, but regular arrays of dots.        
Until recently the visually based methods have dominated so that visual inspection is assumed and is not usually mentioned explicitly. Recent automatic methods are typically just modifications of the visual methods to allow simple optical sensors to monitor spatial variations in optical density. Measurement of individual dot positions, although fundamental, is quite unreliable due to the large variations in dot shape, position and size. There is also the difficulty of unambiguously locating isolated dots in large regions on the medium being printed upon.